Apparatus for producing ions of a given element



Oct. 20, 1959 R; BERNAS ErAL 2,909,697

APPARATUS FOR PRODUCING IONS OF A GIVEN ELEMENT Filed March 23, 1956 2Sheets-Sheet 1 Oct. 20, 1959 B R ET'AL 2,909,697

APPARATUS FOR PRODUCING IONS OF A GIVEN ELEMENT 2 Sheets-Sheet 2 FiledMarch 23, 1956 q Q iezaeqaza ze United States Pa C APPARATUS FORPRODUCING IONS OF A GIVEN ELEMENT Ren Bernas and Jean Druaux, Paris,France, assignors to Commissariat a lEnergie Atomique and CentreNational de la Recherche Scientifique, Paris, France, scieties of FranceApplication March 23, 1956, Serial No. 573,389 Claims priority,application France March 26, 1955 9 Claims. (Cl. 313-63) The presentinvention relates to apparatus for producing ions of a given element,either refractory or not.

Its chief object is to permit the obtainment of ions from elements whichmay be in the solid state, without requiring a high temperature.

A number of apparatus make use of ion beams, in particular ionaccelerators and, more especially, electromagnetic isotope separators.In such apparatus it is necessary to produce intensive beams of ionizedparticles of a given element.

It is therefore necessary to bring the bodies to be ionized into a stateof atomic division.

Up to the present time, the most commonly used solutions for obtainingthis result and forming sources of ions consisted in raising thetemperature of the element to be ionized or of one of its compounds,such as a halide, in order to obtain a vapor tension.

The means for obtaining this result vary with the order of magnitude ofthe temperature necessary for obtaining this vapor tension (which mayrange from 20 to 3000 C.).

For instance, use is made of resistance furnaces in order to vaporizeelements having a vapor tension of moderate value and stable compoundsof elements having a low vapor tension, whereas use is made ofelectronic bombardment furnaces to vaporize refractory elements thecompounds of which, such as halides, having a high vapor tension, areunstable at the temperature necessary for the operation of said ionsources.

These known solutions are complicated and they are not of a generalapplication. Furthermore, in most cases, the use of a compound insteadof the element itself reduces the yield of the ion stream extracted fromthe element, due to the dissociation of the molecules by electronicimpact in such ion sources.

On the other hand, there is a phenomenon known as cathode disintegrationwhich is well known and which consists in the bombardment of a cathodeby positive ions of some elements or compounds of these elementsaccelerated up to speeds higher than one hundred electron-volts, so asto knock out the atoms of said cathode and thus to release particles ofthe element of which said cathode is made, whatever be the temperature.

Our invention consists essentially in subjecting a probe electrode madeof the element of which a beam of ions is to be produced by ionsaccording to the above stated cathode disintegration process, theparticles thus knocked out from this probe electrode whatever be itstemperature being ionized in an electric discharge of any suit abletype, then extracted by means of an accelerator electric field in theform of an intensive ion beam which contains an important amount of theelement of which the probe electrode is made.

Preferably, the same means are used for producing the ions which are tobombard the probe electrode and for ionizing the particles emitted fromsaid electrode,

2,909,697, Pin ed, -29.

which means may be an electronic discharge from a cathode produced in amagnetic field or a high frequency discharge.

Preferred embodiments of the present invention will be hereinafterdescribed with reference to the accompanying drawings given merely byway of example and in which:

Fig. 1 is a perspective view of an apparatus-according to our invention,this view being an exploded one so as to show the elements of theapparatus at a distance from one another.

Fig. 2 shows on an enlarged scale and in sectional view the main portionof the apparatus of Fig. 1.

Fig. 3 is a front view of another embodiment ofour invention to beapplied with an electro-magnetic isotope separator.

Fig. 4 is a sectional View on the line IVIV of Fig. 3.

Fig. 5 is a sectional view on the line VV of Fig. 3', said Fig. 5further showing one of the elements of the device for extracting the ionbeam.

Fig. 6 is a diagrammatical view relative to another embodiment.

In an apparatus as illustrated by Fig; l, the parts are enclosed in acommon envelope (not shown on the drawings) containing an atmosphere ofa rare gas or of a vapor (chlorine, nitrogen, etc., or even the vapor ofthe element of which the probe electrode is made) under a pressurevarying, according tothe nature of said gas or vapor, from anapproximate vacuum to some hundreds of atmospheres.

These parts include a casing 1, a hot cathode 2 at a a potential V (orany other suitable means for emitting electrons), an anode 3 at apotential V a probe electrode 4 at a potential V constituted by theelement of which is to be made the ion beam to be obtained. A magneticfield illustrated by arrow H is produced along the axis of casing 1, forinstance by an electro-magnet only the poles 5 and 6 of which are shown.Two electrodes 7 and 8 at respective potentials equal to V and V areplaced, the first one 7 in the immediate vicinity of the source of ionsand the second one 8 at some distance from the first one. The ion beamthat is obtained is shown at 9.

In the case of a high frequency discharge, cathode 2 is no longernecessary, nor the magnetic field H.

The operation of this apparatus is as follows:

Cathode 2 produces electrons which are accelerated by anode 3 andfocalized along lines of force of the tive space charge density in eachof its volume elements and by the fact that the constant potential ofthis plasma is always close to that of the most positive electrode.

On the other hand, the plasma 10 is limited, in the vicinity of anyelectrode present therein, by a region of very small size called sheathin which there is a potential gradient between the electrode and theplasma, so that the potential of a probe electrode has no materialinfluence, except in the region corresponding to this sheath.

If there is then introduced into the discharge a probe electrode whichis negatively biased with respect to the plasma so that the positiveions of the plasma which pass through the sheath then reach said probeelectrode with a predetermined energy, the impact of these ions on theprobe electrode will release atoms of the element of which saidelectrode is made.

According to our invention, a portion of the atoms knocked out from theprobe electrode as a consequence of this bombardment is ionized in anelectric discharge, which is advantageously the same as that which hasproduced the plasma but which might be different without departing fromthe principle of this invention. These ionized particles are thenextracted in the form of an ion beam 9 by a suitable device constitutedby electrode 7, placed at a potential V, close to that of plasma l, andby electrode 8 at a negative potential V Merely by way of example, Wegive hereinafter some values of potentials which may be used in a sourceof ions according to the present invention.

Potential V of the cathode: about -l00 volts;

Potential V of the anode, the most positive potential, is taken asorigin of the potentials: it is therefore V =0; it is the potential ofthe plasma;

Potential V of the probe electrode: from --200 to l000 volts (currentfrom 1 to 2 amperes);

Potential V, of electrode 7: equal to V i.e. equal to 0; and

Potential V of electrode 8: from l(),000 to 50,()O0 volts.

The probe electrode 4 may be in the solid or liquid state, and it may bea conductor or semi-conductor; it may even be made of an insulatingmaterial.

It should be well understood that the primary ions, that is to say theions which serve to bombard the probe electrode, might be obtained inany other way, without departing from the principle of our invention.For instance, we might form plasma 10 by means of a high frequencydischarge, or make use, in order to disintegrate the probe electrode, ofa beam of primary ions of any origin whatever.

Figs. 3, 4 and 5 relate to a preferred embodiment of our invention,applicable in particular as a source of ions for an electrode-magneticisotope separator. Of course, this apparatus is not limited to saidapplication.

In this construction, the electrons are emitted by a hot filament 2carried by two supports and 16. Anode 3, which serves to acceleratethese electrons, is of cooled copper and it is biased to some hundredsof volts with respect to filament 2. The axial magnetic field H is ofsome hundreds or some thousands of Gauss; the electro magnet whichproduces this field is not shown. The discharge is maintained either bythe element itself which emits a vapor, or by an inert or chemicallyactive gas, or again by a vapor.

In this construction, the probe electrode is constituted by a sheet 12of the element to be ionized or of a compound or alloy thereof,supported inside a hollow cylinder 11 made of graphite. The form of theraw material that is used is not critical, which greatly facilitatesconstruction of the apparatus. This probe electrode is fixed in casing lby a support 13 secured in an insulator 14.

Due to this arrangement and to the distribution of potential imposed bythe probe electrode, the plasma of the discharge is concentrated insidecylinder 11 and the probe electrode is disintegrated by the impact ofthe positive ions on its inner face.

A portion of the atoms knocked out from the probe electrode, afterhaving been ionized in the discharge, is accelerated by a suitabledevice including the electrode 7 shown on Fig. 5 and extracted from thesource in the form of an ion beam toward the electro-magnetic separator.Another portion is recondensed upon the inner walls of the probeelectrode, which permits its subsequent reemission by the same process.

In order to reduce to a minimum the losses of disintegrated material,the open surface of the cylinder is limited to the slot for the outflowof the ions and to the side aperture on the side of the hot cathode 2necessary for the inflow of the electrons into the discharge. Thisarrangement has the advantage of increasing the chiciency of ionizationof the element that is considered, and to protect the insulator 14 ofthe probe electrode against metallizing.

Fig. 6 illustrates the case of high frequency being used, as abovestated, for producing the discharge. A hollow cylinder 11 contains asheet 12 of the element to be ionized and the plasma is created insidethis cylinder by a high frequency discharge taking place in a winding 17fed from a source 18. This discharge may take place with or without anaxial magnetic field H and the frequency may average megacycles.

The ion producing apparatus according to the invention work quite aswell with refractory metals or with non-refractory metals.

By way of example, this apparatus has been used for extracting ion beamsfrom elements such as molybdenum, palladium, iron and copper; thesebeams contained from 20 to 100% of the disintegrated element.

Comparison of the characteristics of the conventional discharge and of adischarge making use of the cathode disintegration phenomenon showsanother advantage of the apparatus according to our invention. This isdue to the fact that with the same element and with a constant beam ofions extracted in both cases, there is an increase of the resolutionpower when recording the same mass spectrum on the electro-magneticseparator.

In a general manner, while we have, in the above description, disclosedwhat we deem to be practical and efficient embodiments of our invention,it should be well understood that we do not wish to be limited theretoas there might be changes made in the arrangement, disposition and formof the parts without departing from the principle of the presentinvention as comprehended within the scope of the accompanying claims.

What we claim is:

1. An apparatus for producing ions of a given element which comprises,in combination, an envelope filled with a gaseous atmosphere, electronicmeans for producing a discharge in said atmosphere, an electrode in saidenvelope located in the portion of said discharge where a plasma isformed, said electrode being at a potential more negative than saidplasma, whereby positive ions of said gaseous atmosphere are made tobombard said electrode and to knock out particles of said element fromsaid electrode, said particles being ionized by said discharge producingmeans, and means for extracting a beam of said ionized particles fromsaid envelope.

2. An appartus according to claim 1 in which said plasma is at zeropotential.

3. An apparatus for producing ions of a given element which comprises,in combination, an envelope filled with a gaseous atmosphere, a hotcathode for producing an electric discharge in said gaseous atmosphere,an anode for accelerating the flow of electrons from said cathode, meansfor producing in said gaseous atmosphere an axial magnetic fieldfocalizing the electrons emitted by said cathode, an electrode made ofsaid element located in said atmosphere, means for placing saidelectrode at a potential suitable to enable bombardment thereof by thepositive ions of said gaseous atmosphere produced by said discharge,whereby particles of said element are knocked out from said electrode,said particles being ionized by said discharge, and means for extractinga beam of said ionized particles from said envelope.

4. An apparatus according to claim 3 in which said means for extractingthe beam of ionized particles consist in two electrodes at least one ofwhich is at a very high negative potential.

5. An apparatus according to claim 3 in which said electrode is in thesolid state.

6. An apparatus according to claim 3 in which said electrode is in theliquid state.

7. An apparatus for producing ions of a given element which comprises,in combination, an envelope filled with a gaseous atmosphere, electronicdischarge means for producing an electric discharge in said gaseousatmosphere, an electrode made of said element located inside saidatmosphere, means for placing said electrode at a potential suitable toproduce bombardment thereof by the positive ions of said gaseousatmosphere due to said discharge, whereby particles of said element areknocked out from said electrode, said particles being ionized by saiddischarge producing means, and two electrodes, at least one of which isat a very high negative potential, for extracting a beam of said ionizedparticles from said envelope.

8. An apparatus for producing ions of a given element which comprises,in combination, an envelope filled with a gaseous atmosphere, electronicdischarge means for producing an electric discharge in said gaseousatmosphere, an electrode consisting of a cylindrical sheet at leastpartly constituted by the element to be ionized located inside saidatmosphere, a hollow graphite cylinder disposed to contain said sheet,insulating means for supporting said cylinder, means for placing saidelectrode at a potential suitable to produce bombardment thereof byReferences Cited in the file of this patent UNITED STATES PATENTS2,498,841 King Feb. 28, 1950 2,642,535 Schroeder June 16, 1953 2,677,061Wilson Apr. 27, 1954 2,677,771 Turner May 4, 1954 2,712,097 AuwarterJune 28, 1955 2,798,181 Foster July 2, 1957

1. AN APPARATUS FOR PRODUCING IONS OF A GIVEN ELEMENT WHICH COMPRISES,IN COMBINATION, AN ENVELOPE FILLED WITH A GASEOUS ATMOSPHERE,ELECTRONICMEANS FOR PRODUCING A DISCHARGE IN SAID ATMOSPHERE, AN ELECTRODE IN SAIDENVELOPE LOCATED IN THE PORTION OF SAID DISCHARGE WHERE A PLASMA ISFORMED, SAID ELECTRODE BEING AT A POTENTIAL MORE NAGATIVE THAN SAIDPLASMA, WHEREBY POSITIVE IONS OF SAID GASEOUS ATMOSPHERE ARE MADE TOBOMBARD SAID ELECTRODE AND TO KNOCK OUT PARTICLES OF SAID ELEMENT FROMSAID ELECTRODE, SAID PARTICLES BEING JONIZED BY SAID DISCHARGE PRODUCINGMEANS, AND MEANS FOR EXTRACTING A BEAM OF SAID IONIZED PARTICLES FROMSAID ENVELOPE.